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Repair of a uracil‐guanine base pair in DNA has been reconstituted with the recombinant human proteins uracil‐DNA glycosylase, apurinic/apyrimidinic endonuclease, DNA polymerase beta and DNA ligase III. The XRCC1 protein, which is known to bind DNA ligase III, is not absolutely required for the reaction but suppresses strand displacement by DNA polymerase beta, allowing for more efficient ligation after filling of a single nucleotide patch. We show that XRCC1 interacts directly with DNA polymerase beta using far Western blotting, affinity precipitation and yeast two‐hybrid analyses. In addition, a complex formed between DNA polymerase beta and a double‐stranded oligonucleotide containing an incised abasic site was supershifted by XRCC1 in a gel retardation assay. The region of interaction with DNA polymerase beta is located within residues 84–183 in the N‐terminal half of the XRCC1 protein, whereas the C‐terminal region of XRCC1 is involved in binding DNA ligase III. These data indicate that XRCC1, which has no known catalytic activity, might serve as a scaffold protein during base excision‐repair. DNA strand displacement and excessive gap filling during DNA repair were observed in cell‐free extracts of an XRCC1‐deficient mutant cell line, in agreement with the results from the reconstituted system.

Spermatogenesis is one of the most complex and longest processes of sequential cell proliferation and differentiation in the body, taking more than a month from spermatogonial stem cells, through meiosis, to sperm formation. The whole process, therefore, has never been reproduced in vitro in mammals, nor in any other species with a very few exceptions in some particular types of fish. Here we show that neonatal mouse testes which contain only gonocytes or primitive spermatogonia as germ cells can produce spermatids and sperm in vitro with serum-free culture media. Spermatogenesis was maintained over 2 months in tissue fragments positioned at the gas-liquid interphase. The obtained spermatids and sperm resulted in healthy and reproductively competent offspring through microinsemination. In addition, neonatal testis tissues were cryopreserved and, after thawing, showed complete spermatogenesis in vitro. Our organ culture method could be applicable through further refinements to a variety of mammalian species, which will serve as a platform for future clinical application as well as mechanistic understanding of spermatogenesis.

ARTICLEMen have a substantially higher risk of bladder cancer than women ( 1 ). Excessive exposure of men to cigarette smoke and industrial chemicals, both of which include amines, has been suggested to result in the development of bladder cancer ( 2 ). However, sexrelated differences in the risk of bladder cancer have been shown to persist in the absence of exposure to known carcinogenic factors ( 2 ). In animal experimental models, males are more likely than females to develop bladder cancer induced by certain chemical carcinogens (e.g., aromatic amines, such as N -butyl-N -(4-hydroxybutyl) nitrosamine [BBN]) ( 3 ). In contrast, a recent study ( 4 ) showed that certain other carcinogens, such as the arsenical metabolite dimethylarsinic acid, are more toxic to the female bladder than the male bladder in rats. This finding is consistent with epidemiologic evidence suggesting that women are more susceptible to arsenic-induced bladder cancer than men ( 5 ). Thus, the basis for the sex-specific difference in bladder cancer incidence is not understood.A potential mediator of sex-specifi c differences is the androgen receptor (AR). The AR, a member of the nuclear receptor superfamily, is a ligand-dependent transcriptional factor that mediates the biologic effects of androgens ( 6 , 7 ). Expression of the AR has been detected in normal bladder epithelium ( 8 ) and in bladder carcinomas from both male and female patients ( 9 ). However, little is known about AR function in the bladder or about androgen metabolism in the bladder urothelium. Early studies ( 10 ) showed that levels of cytochrome P450 CYP4B1, which is present at higher levels in male bladder than female bladder and activates Affiliations of authors:

DNA polymerase β (Polβ) has been implicated in base excision repair in mammalian cells. However, the physiological significance of this enzyme in the body remains unclear. Here, we demonstrate that mice carrying a targeted disruption of the Polβ gene showed growth retardation and died of a respiratory failure immediately after the birth. Histological examination of the embryos revealed defective neurogenesis characterized by apoptotic cell death in the developing central and peripheral nervous systems. Extensive cell death occurred in newly generated post-mitotic neuronal cells and was closely associated with the period between onset and cessation of neurogenesis. These findings indicate that Polβ plays an essential role in neural development.